Antibiotics and other growth promoting antimicrobials have been used as animal feed additives for over 45 years. The economic benefits to the animal producer include; improved feed conversion, reduced mortality, and greater resistance to disease challenge (Jukes, 1972; Guest, 1976; Hays, 1978).
A review of the literature conducted in 1996 indicated that in 12,153 trials, the addition of antimicrobial growth promotants to the diet increased production 72% of the time (Rosen, 1996).
Today more than 50% of all antibiotics produced are used in animal feeds. However, because of the potential risk to human health many consumer groups, government officials and researchers promote the elimination of sub-therapeutic doses of antibiotics in animal feeds, in an attempt to reduce or eliminate the antibiotic resistant bacterial populations. Fecal shedding of antibiotic resistant bacteria is one of the primary mechanisms of transfer of antibiotic resistance.
Enteric diseases are the greatest cause of illness and death in calves (NAHMS, 1996).
Greater than 50% of dairy calves may experience diarrhea caused by enteric diseases (Franklin et al., 1998). Producers often treat calves with antibiotics therapeutically even though most of the causative agents for diarrhea are not responsive to antibiotics.
Additionally, most commercial milk replacers contain sub-therapeutic levels of antibiotics in an attempt to prevent enteric diseases.
Antibiotic Use in Dairy Applications
Although direct evidence for a link between sub-therapeutic antibiotic use and human infection is lacking, several studies have implicated infections in humans to multiple drug resistant strains of bacteria in animals.
An epidemic of resistant Salmonella heidelberg in Connecticut was traced to calves on a dairy farm (Lyons, et al., 1980). Eighteen people in the midwest became ill when infected with multiple-drugresistant Salmonella newport traced to a beef herd in South Dakota (Holmberg et al., 1984).
Separate outbreaks of Salmonella typhimurium in Arizona and Canada were traced to multiple-drug-resistant bacteria in raw milk (Tacket et al., 1985; Bezanson et al., 1983). Finally, an outbreak of S. newport in 45 patients in California was linked to beef from dairy farms (Spika et al., 1987).
Even the perception that contaminated dairy products or meat from ruminant animals could endanger humans may hinder consumption. Therefore, the dairy industry must be proactive in searching for alternatives to antibiotics. A frequent area of use of sub-therapeutic antibiotics in the dairy industry is in the formulation of milk replacers.
Martel and Coudert (1993) suggested that the emergence of antibiotic resistant organisms was greater in young calves compared with older cattle, reasoning that drug use is greater with young calves because of greater susceptibility to bacterial diseases compared to older animals. In addition, the transfer of antibiotic resistance from cows receiving antibiotics to nursing calves has also been examined.
Development of antibiotic resistant microorganisms was investigated in calves fed milk from cows treated for mastitis (Wray et al., 1990). Calves that received milk from cows treated with antibiotics for mastitis developed bacteria that were resistant to streptomycin whereas calves in the group fed milk replacer without antibiotics did not shed antibiotic resistant organisms into the environment.
Other studies have reported the existence of antibiotic resistant organisms in calves. Hariharan et al. (1989) isolated E. coli that was resistant to potentiated sulfonamide (a combination of trimethoprim and sulfamethoxazole) from fecal samples of calves with diarrhea. Also, all the E. coli isolates were resistant to tetracycline, and most were resistant to neomycin and ampicillin as well.
A summary of studies from several countries indicates that multiple-drug resistant E. coli is widespread in calves with enteritis (National Research Council, 1999).
Microbiology of the neonate
The newborn animal is most susceptible to enteric pathogens due to the development of the neonatal gastrointestinal microflora. Ruminants, like other mammals, are born devoid of indigenous intestinal microorganisms.
During transport through the vagina some bacteria are acquired by the neonate. This route may provide the initial inoculum of the newborn gastrointestinal tract, since predominant bacteria in the vagina include lactic acid bacteria. After passage through the birth canal, the neonate may become contaminated and subsequently colonized by fecal organisms such as Escherichia coli.
These events may help to explain the initial microflora of the neonate described by Smith (1965). In his work, E. coli, Clostridium perfringens and streptococci are described as the first organisms to be found in the neonate followed closely by lactobacilli. Lactobacilli are slower to grow than the other organisms, but it has been documented that a noticeable Lactobacillus population corresponds to a drop in stomach pH (Newman and Jacques, 1995).
One threat to the neonatal ruminant stems from the fact that the initial gastrointestinal tract pH is near neutrality. Since the microbial population is in transition and extremely sensitive at this time, it is susceptible to colonization from pathogenic bacteria.
For this reason, enteric diseases are the greatest cause of illness and death in calves (NAHMS, 1996). Greater than 50% of dairy calves may experience diarrhea caused by enteric diseases (Franklin et al., 1998). Greater than 60% of calf deaths are attributed to diarrhea or other digestive problems (NAHMS, 1996). These problems occur despite the fact that most commercial milk replacers contain sub-therapeutic levels of antibiotics in an attempt to prevent enteric diseases.
Direct Fed Microbials and CE Cultures in Pre-ruminants
One alternative to antibiotics that needs to be explored is the development of a competitive exclusion product for use with calves to help prevent colonization with pathogenic microorganisms.
Direct fed microbials (DFMs) are currently available for calves but are primarily lactic acid bacteria and have not been consistently effective in preventing pathogenic infections. Attachment and colonization of the GI tract is an important factor in successful competitive exclusion of pathogens but does not appear to be the only mechanism to lower infection rate.
Several studies have examined the ability of microorganisms to colonize the GI tract (Savage, 1972; Savage et al., 1968; Davis and Savage, 1972). In chickens, it was shown that only Lactobacillus strains of avian origin were able to colonize crop epithelial cells (Fuller and Brooker, 1974).
However, reductions in pathogens have been associated with both adhering and non-adhering DFM bacteria. Jonsson and Olsson (1985) isolated Lactobacillus strains from the GI tract of calves that were shown in vitro to adhere to squamous epithelial intestinal cells. Efficacy of these strains in calves demonstrated no benefit in terms of performance and health of calves receiving supplementation.
Another study examined a nonviable L. bulgaricus fermentation product in calves and found improvements in weight gain and feed consumption without affecting fecal lactobacilli or coliform concentrations (Schawb et al., 1980). Reductions in scouring by calves have been reported for both Lactobacillus and Streptococcus (Enterococcus) supplements (Bechman et al., 1977; Fox, 1988; Maeng et al., 1987) which were not examined for their ability to colonize the GI tract.
Other investigators have found improved body weight gain, feed conversion and diarrhea with supplementation of either Bifidobacterium pseudolongum or Lactobacillus acidophilus (Abe et al., 1995). Our own work in pigs demonstrated alterations in fecal and intestinal lactic acid bacteria and coliforms when supplemented with a DFM containing Enterococcus faecium and Lactobacillus acidophilus (Newman, 1990).
The variability in results that have been noted with DFM usage in calves may be based upon a number of factors including product quality, suitability of strains used and number of strains used.
To date, the only such CE culture exists for poultry.
Mannan oligosaccharide and calf and ruminant production
Mannan oligosaccharide (MOS) is a complex carbohydrate consisting of polymers of mannose and mannose derivatives. Complex carbohydrates have risen to a prominent research topic with the realization that distinct carbohydrate structures can have specific biological activities.
The biological diversity of these compounds can be easily demonstrated by examining the difference between alpha and beta-bonded 1-4 glucose units. When these two glucose units are bound in the alpha configuration, the compound, amylose, is easily degraded by starch degrading enzymes found in saliva. Conversely, beta-bonded 1-4 glucose represents cellobiose, a compound that is not degraded by any mammalian enzyme system.
This example only looks at the difference in biological activity of the same two glucose molecules bound together at the same site with only a difference in the type of bond between these glucose units. One need only imagine the diverse nature of carbohydrate chemistry to see that the opportunities for novel compounds with unique biological activity. Carbohydrates and oligosaccharides are also now being utilized for their role in nutrition and immunity.
Carbohydrates are important surface entities of many animal cells, they project from the cell surface and form the antigenic determinants of certain cell types. Bacteria (including pathogens) recognize these sugars and have receptors which allow them to attach, colonize and, in the case of pathogens cause disease in the animal. Mannose specific lectins (protein fimbriae on the bacterial surface), are utilized by many gastrointestinal pathogens as a means of attachment to the gut epithelium (Mirelman and Ofek, 1986).
One way to prevent pathogens from causing disease is to prevent them from attaching to the epithelial cells in the gut. A complex sugar called mannan oligosaccharide has been successfully used to prevent this attachment by providing the bacteria a mannose-rich receptor that serves to occupy the binding sites on the bacteria and prevent colonization.
Several studies have been conducted examining the role of mannans and their derivatives on binding of pathogens to epithelial cells in the GI tract. E. coli with mannose-specific lectins did not attach to mammalian cells when mannose was present (Salit and Gotschlich, 1977). Spring and coworker (2000), used a chick model to demonstrate that MOS could significantly reduce the colonization of Salmonella and E. coli.
Animal trials in other species show similar benefits in reducing pathogen concentrations. In a number of calf trials, calves receiving MOS in milk replacer formulations had lower fecal coliform and E. coli concentrations than calves receiving unsupplemented milk replacer (Table 1).
|Table 1. Effect of MOS supplementation on fecal bacterial populations.|
||Control (log CFU/g)
||Jacques and Newman, 1994|
||Newman, et al., 1993|
Reductions in fecal scouring and improved fecal consistency have also been noted in a number of trials (Dildey, et al., 1997; Nippei, 1996; Heinrichs, 2001). In addition, reductions in pathogen concentrations do not seem to be limited to the pre-ruminant.
In a field trial situation using heifers, the number of Salmonella positive heifers was dramatically reduced when MOS was added to the diet. Unlike many feed additives, MOS is not affected by heat. In laboratory tests, heating MOS to 121oC for 20 minutes, had no adverse effects on any of the parameters examined.
MOS Effects on Immune Function
In addition to positive responses in terms of pathogen control in a number of species with MOS supplementation, examination of immune function in a number of species ranging from turkeys to dogs and pigs indicate that immunoglobulin levels can be influenced by MOS supplementation. Savage and coworkers found bile IgA and plasma IgG levels to be greater in birds supplemented with MOS compared to unsupplemented birds (1996).
In sows receiving MOS 14 days pre-farrowing and throughout lactation, higher concentrations of colostrum IgG and IgM were noted compared to unsupplemented sows (Table 2). In addition, the piglets from supplemented sows were significantly heavier at weaning. These data have enormous implications for the dairy industry. Improved passive immunity can decrease neonatal infection rates and improve growth performance as a result of a healthier calf. A similar trial in cows is under way.
|Table 2. Effects of MOS supplementation of sows in colostrum immunoglobulin status (all immunoglobulin concentrations expressed in mg/dl).|
|Newman and Newman, 2001.|
The cellular immune system has also been shown to be affected by MOS supplementation. Macrophage activity was enhanced in mice receiving MOS supplementation compared to unsupplemented animals. Macrophages represent a nonspecific first line of defense in many parts of the animal. The lungs, digestive tract, blood and tears are all areas where macrophage activity is a principle part of host immune function.
It is not surprising then to find that calves receiving milk replacer supplemented with MOS had a lower incidence of respiratory disease than unsupplemented calves (Newman et al., 1993). As with other species, respiratory problems of the young calf are influenced by a number of factors including, dust, humidity, bacteria and viral challenge and immune status. Enhanced macrophage activity may lead to more efficient removal of respiratory stressors.
What can’t be lost in all the data supporting the benefits of MOS on gastrointestinal health and immune function is the improvement in calf performance with the inclusion of MOS in the milk replacer or starter diet. An overall improvement in performance of over 17% has been observed in summarizing over 12 trials conducted with MOS from Alltech, Inc.
Improving the health and immune status of the animal through nutritional sources is an area prime for pioneering research. Mannan oligosaccharide is a complex carbohydrate which has been shown in a variety of species to provide benefits to supplemented animals via two basic mechanisms:
(1) Agglutination of bacteria which may cause enteric disease in the animal. Trials in species ranging from dogs and poultry, to pigs and ruminants have demonstrated reductions in pathogens such as E. coli, Salmonella, and clostridia.
(2) Modulation of immune function. Again, trials in a variety of species demonstrate that immunoglobulin concentrations can be altered in the colostrum, serum and organs. Vaccination titers have also demonstrated improvements with MOS inclusion in the diet. Nonspecific cellular immunity has also been positively influenced in studies examining macrophage activity.
In addition, improvements in overall gastrointestinal tract function have been noted and expressed as improved gain.
In an era that is under the threat of withdrawal of antibiotic supplementation of livestock feeds, alternatives that provide cost-effective benefits similar to traditional growth promotants demand a closer examination from producers. Mannan oligosaccharides have been shown to reduce disease incidence, and improve calf performance.
Author: Kyle Newman, Ph.D.
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